1. Field of the Invention
The present invention relates to an image-forming apparatus and control method thereof and, more particularly, to an image-forming apparatus which comprises a plurality of sheet-feed units and a control method thereof.
2. Description of the Related Art
An image-forming apparatus such as a copying machine, printer, facsimile apparatus, or the like has a sheet container (sheet-feeding tray) which stores sheets, and transfers a sheet from the sheet container to an image-forming unit using a sheet-supply unit. An image is formed on the sheet which is transferred in this way, and the sheet on which the image has been formed is discharged outside the apparatus. As the sheet-supply unit of such image-forming apparatus, a sheet-supply unit of roller type which transfers a sheet downstream by rotation of sheet-feed rollers is generally used. In this sheet-supply unit of roller type, the surface of each roller is made up of an elastic member such as rubber or the like, and its feeding performance largely depends on the friction coefficient of the roller surface. The feeding performance may decline due to a change in outer shape of each roller, aging of the material of each roller, and a change in friction coefficient of the roller surface due to attachment of paper powder and the like. The sheet-supply unit of this type cannot support high-speed feeding and various sheets with different surface conditions.
In order to solve these problems, Japanese Patent Laid-Open No. 6-199437 has proposed an air-feeding apparatus which adopts an air-separation system. This air-feeding apparatus comprises handling means for handling upper sheets by blowing air to the end portion of sheets stacked in a sheet-stack unit, and absorptive transfer means for absorbing and transferring an uppermost sheet onto a conveyor belt.
A conventional image-forming apparatus has a plurality of sheet containers so as to form images on various types of sheets, and can store sheets of various sizes for respective sheet containers. Also, by storing sheets of an identical size in the plurality of sheet-supply units, an image-forming apparatus which stores sheets in large quantities in itself and can form images in large quantities is currently popular.
When the image-forming apparatus having the plurality of sheet-feed units uses air-feeding apparatuses, the following problems are posed.
The air-feeding apparatus equipped in each sheet-feed unit has means for handling upper sheets by blowing air to the end portion of a bundle of sheets, and means for absorbing an uppermost sheet on a transfer belt, and these means use pneumatic pressure. Upon using the pneumatic pressure, a time delay is produced until the pneumatic pressure acts on sheets in each sheet-feed unit even by extracting/suctioning air so as to attain feeding. Causes of such delay include the duct length, the switching time of an on-off valve in the duct used to switch air for each sheet-feed unit, a delay of a handling time due to the weight of sheets, and the like. In this way, the air-feeding apparatus suffers a low throughput of jobs due to a time delay until the pneumatic pressure acts on sheets even upon starting the sheet feeding.
Japanese Patent Laid-Open No. 2002-40881 describes a technique that checks if sheet containers other than that which has caused an “out of paper” error store sheets with an identical size, and continues to feed sheets from the sheet container without stopping the feeding operation if such a sheet container is found. With this function, the operation stop time due to the “out of paper” error is shortened, and print job efficiency is enhanced.
Japanese Patent Laid-Open No. 5-286590 has proposed an air-feeding apparatus which eliminates exchange delay time of sheet-feed units by applying air to two feeding apparatuses all the time. In this Japanese Patent Laid-Open No. 5-286590, one air extraction device and air supply device distribute extracted air and supply air to a plurality of trays. By setting a pressure that allows feeding sheets upon opening on-off valves of two trays, on-off control means executes valve control to always open the two on-off valves. With this control, since the two on-off valves are always open, no or little pneumatic pressure variation occurs even when sheets are fed from both of the two trays. Thus, upon exchange of sheet-feed units, the pneumatic pressure on the tray side can be quickly changed to a value required for the operation. In this manner, an air-feeding apparatus which can prevent the throughput of print jobs from lowering by eliminating any exchange loss of sheet-feed units has been proposed.
As described above, air feeding can meet a higher-speed image-forming apparatus compared to roll feeding. However, since the air feeding requires much time until it becomes ready to feed, a time required until first printing (the first print output after the image-forming apparatus accepts a job execution instruction) is prolonged. On the other hand, roll feeding cannot support the highest print speed of a printer engine in case of continuous print and feeding.